A fine powder of a pure metal or an alloy is used as a material for a sintered alloy, an additive for metal refining, an additive for a paint, a material for fillers for a plate grid of a battery and in a wide variety of other applications.
Heretofore, many processes for manufacturing a fine powder of a pure metal or an alloy (hereinafter generally referred to as "metal") are known, but among these, the mechanical pulverizing process, the molten metal pulverizing process, the reduction process and the electrolytic process have been put into practical use in an industrial scale. Particularly, the molten metal pulverizing process is widely applied because of the capability of pulverizing a metal more easily and more easily and more efficiently than the pulverization of a solid metal.
The molten metal pulverizing process has however a disadvantage in that it is difficult to manufacture a fine powder because of the very short time for pulverization of molten metal. This process may roughly be classified into the following three types:
(a) The shotting process which comprises dripping a molten metal in the form of droplets into water through a small hole and solidifying said molten droplets by cooling thereby granulating said molten metal; PA1 (b) The graining process which comprises cooling and solidifying a molten metal while intensively stirring in the atmosphere and utilizing the oxidization of said metal during this period, thereby granulating said molten metal; PA1 (c) The atomizing process which comprises causing a molten metal to flow out through a small hole in the form of a tiny stream, applying a water jet or a centrifugal force to said stream to finely disperse said molten metal and simultaneously cool and solidify said dispersion, thereby granulating said molten metal.
The shotting process described in (a) above is used principally for the granulation of lead, tin, zinc, aluminum, copper and copper alloys. A metal powder obtained by this process comprises coarse shots having the shape of spheres or drops, and it is difficult by this process to manufacture a powder with a particle size of up to 1 mm.
The graining process described in (b) above is used principally for the granulation of zinc and aluminum. A metal powder obtained by this process comprises relatively coarse grains having an irregular or drop shape with their surfaces covered with oxides, and most of the grains fall into a particle size range of from about 20 to about 100 mesh (from about 833 to about 147 .mu.m).
The atomizing process described in (c) above is applied for the pulverization of almost all metals other than those particularly susceptible of oxidation and with an extremely high melting point. With this process, it is possible, under appropriately selected conditions, to manufacture a metal powder with any of various shapes such as sphere and drop as well as with any of various grain sizes ranging from a fine powder of several tens of .mu.m to a relatively coarse powder of several hundreds of .mu.m. However, the surfaces of the metal powder particles obtained by this process are often covered with oxides, and in addition, the molten metal tends to cool and solidify rapidly because of the high cooling ability of the atomizing medium. As a result, the atmospheric gas either entrained into the metal powder or dissolved in the molten metal is relieved not completely during the solidification of the molten metal, thereby often causing pores in the metal powder obtained.
Also, a metal powder obtained by the mechanical pulverizing process using such an equipment as a ball mill hardens because of the residual strain remaining in it and inevitably contains impurities mixed in it. In this process, furthermore, the metal powder obtained takes the form of either fish scale, flat plate or dish, and hence, it is difficult to manufacture a metal powder having a uniform shape and grain size and it takes much time for pulverization.
A metal powder obtained by the reduction or electrolytic process is poor in fluidity as it presents a dendritic shape and is not suitable for use as the material for sintered alloys based on the powder metallurgy process.